The Nordre StrÃ¸mfjord shear zone and the Arfersiorfik quartz diorite ...

More documents

Recommendations

Info

51°30' 51° B 15705 Inland 15703 Ice 68° Tunertoq Arfersiorfik 12702 Sarfaarsuk Nuussuaq 18702 KS Orlerfik Akunnaaq A 15702 05701 05702 01703 03706 inner Arfersiorfik 01704 Polonia 10 km Nuersorfik 13705 13704 50°30' Gletscher Arfersiorfik Quartz Diorite (AQD), mapped and inferred Siliceous gneisses (probably orthogneisses), mapped and inferred Major outwash plains Highly deformed, chemically altered and pegmatite-invaded part of the AQD ('pencil gneiss'), mapped and inferred Supracrustal schist units, mapped and inferred Fault (inferred) Structural trend line Fig. 2. Simplified geological map of the inner Arfersiorfik region, showing the authors’ 2002 field work along full-coloured coasts (see also Fig. 1). Compare the mapped geology with the magnetic patterns on Fig. 7, shown at the same scale. Red lines, structural trend lines mainly from aeromagnetic data. KS, Kangimut Sammisoq (near map centre). Five digit numbers, locality numbers mentioned in the text. Two antiformal folds east of the inferred fault along inner Arfersiorfik are outlined by the AQD contact overlying siliceous gneisses north of localities 05702 and 03706. The axial trace of the intervening synformal fold meets the coast near loc. 01703. Tent symbol, position of camp during 2002 field work. Inland geology near Nuersorfik from van Gool & Marker (2004). thrust nature. Farther to the south, in the area between the heads of Nordre Strømfjord and Arfersiorfik, a thin (up to 500 m) sheet of AQD with tectonic contacts occurs with Archaean siliceous gneisses both above and below the AQD sheet (the Ussuit unit; Manatschal et al. 1998; van Gool et al. 1999). The geometry of this bounding surface is interpreted as outlining a series of folds. Only the northern limb of the northernmost synformal structure is contained within the composite profile of Fig. 3. Folding of the AQD was observed by Henderson (1969) and later elaborated upon by Passchier et al. (1997) and van Gool et al. (1999). At the localities outside the shear zone where the AQD can be seen in contact with the structurally underlying siliceous gneisses (localities 01702, SSE A AQD Homogeneous gneisses with few inclusions neosome increases Three Massive metadolerites coarse-grained AQD 'Hornblende gneiss rich in microcline' of Henderson (1969): lineated and fine grained with abundant pegmatite sheets Amphibolites Granitic orthogneiss Hornblendebearing orthogneiss Gneisses with abundant inclusions NNW B 1 km 01703 01702 (Cumulate) (31 km south) 1st supracrustal unit crustal 2nd supra- unit 3rd supracrustal unit 02701 02702 08703 08704 18702 4th supracrustal unit Nordre Strømfjord shear zone 5th supracrustal unit 12702 Metadolerite locality Mafic enclaves Fig. 3. Schematic profile A–B through the Nordre Strømfjord shear zone in the inner Arfersiorfik from Akunnaaq in the south to Sarfaarsuk in the north (Figs 2, 7). Localities and lithologies have been projected onto the profile plane along the strike direction of the shear zone. Locality numbers mentioned in the text appear beneath the profile. For colour code, consult caption to Fig. 2. 148
B A C D B Fig. 4. Siliceous gneisses between localitiess 17701 and 02702 (Figs 2–3) illustrating progressive deformation towards the first supracrustal unit. A: Coarsely banded biotite gneiss, loc. 17701 (knife 10 cm long). B: Finely banded gneiss with generally steeply oriented foliation, but apparently variable deformation intensity at outcrop scale, loc. 02701 (compass 10 cm long). C: Extreme planar fabric and fine-scale layering in siliceous gneiss near the first supracrustal schist unit, loc. 02702. D: Detail from the same locality with lenticular ultramafic inclusions (knife 10 cm long). 01704, 03706, 05701, Figs 2, 3), the contacts are all tectonised, and with a well-developed, folded planar fabric. Original intrusive features have been obliterated except at loc. 05702 (Fig. 2), where xenoliths of pelitic schist can be seen within the AQD, clearly indicating intrusion of the quartz diorite into a pre-existing metamorphosed sedimentary sequence. From loc. 01702 (Fig. 3) and towards the north, siliceous gneisses become progressively more deformed and contain an increasing amount of granitic material. For a distance of almost 1 km to the south of the first supracrustal schist unit, the gneisses display a subvertical planar fabric and contain abundant layers of leucocratic, in places pegmatitic, material. These gneisses appear highly strained. The transition within these gneisses, from 01701 to 02702 (Fig. 3), is the best exposed transition zone developed in siliceous gneisses in the area that we have visited. The structural development of these gneisses is illustrated in Fig. 4A–C. The first supracrustal unit (Fig. 3) comprises a predominant lithology of mafic schists with calc-silicate and ultramafic lenses. Pelitic schists are subordinate. At locality 08703, a second thin supracrustal unit of pelitic schist (thickness 25 m) occurs along the boundary to the AQD. The gneisses to the north of the first supracrustal unit contain abundant layers of homogeneous amphibolite, 149
Page 1 and 2: The Nordre Strømfjord shear zone a
Page 3: and conversely widens deeper in the
Page 7 and 8: Fig. 6. Pelitic and mafic schists f
Page 9 and 10: 68° 51°30' Tunertoq Arfersiorfik
Page 11 and 12: B A B C D Fig. 8. Extent of litholo
Page 13 and 14: Fig. 11. Peraluminous garnet- and m
Page 15 and 16: farther towards the west-south-west
Page 17 and 18: Sørensen, K. & Winter, J.D. 1989:

The Nordre StrÃ¸mfjord shear zone and the Arfersiorfik quartz diorite ...

Create successful ePaper yourself

Delete template?

Save as template?